In LARP systems, a conversion element, which includes or consists of a phosphor and is arranged at a distance from a radiation source, is irradiated with excitation radiation, in particular an excitation beam (pump beam, pump laser beam), in particular with the excitation beam from a laser diode. The excitation radiation of the excitation beam is at least partially absorbed by the phosphor and at least partially converted into conversion radiation, the wavelengths of which, and thus the spectral properties and/or color, is determined by the conversion properties of the phosphor. By way of example, it is possible in this manner by way of the conversion element to convert blue excitation radiation (blue laser light) into red and/or green and/or yellow conversion radiation (conversion light). A system such as this is also referred to as a laser activated remote phosphor (LARP) system.
Conversion light and any unconverted excitation light together form the used light. If, for example, a blue laser diode having an excitation radiation in the wavelength range of approximately 440 to 470 nm and a conversion element, which for example consists of cerium doped yttrium-aluminum garnet, i.e. Ce:YAG, and converts into conversion radiation in the yellow wavelength range, are used, then, as the proportion of unconverted laser radiation in conversion light decreases, bluish, white-bluish, white, white-yellowish or yellow used light is obtained. The color coordinates of the used light in the automotive field are preferably in the standardized ECE white field in accordance with the guideline ECE/324/Rev. 1/Add.47/Reg.No. 48/Rev.12.
It has been shown here that this blue-yellow color shift of the used light results in what are known as blue-yellow rings in the used light downstream of the conversion element. The central radiation is blue in excess, and the exterior radiation is yellow in excess, with the result that blue-yellow rings form on the emission side of the conversion element if the excitation radiation profile is symmetric. As a result, a blue-yellow ring problem, or color variances of the surfaces to be irradiated (road, wall), which are however undesired in vehicle headlights or in effect lighting, even occur in the provided applications.
Such LARP systems are used for example as light sources in vehicle headlights or projection applications. Suitable optical systems can be provided to focus the excitation radiation onto the conversion element.
It has been shown that a color point path, that is to say a locational dependence of the color point, exists by way of the light-exit surface of the conversion element. Moreover, a color point path can occur as viewed over the viewing angle, i.e. an angular dependence of the color point. If, for example, the LARP system is employed in a vehicle headlight, it is typically combined with optical elements, such as for example reflectors and lenses. Projection of the used light that is inhomogeneous in terms of color into an illumination region thus produces a color point path in the illumination region. The inhomogeneity of the used light in the illumination region is here undesired.